Conventionally, the gear ratio of a gear train, also known as its speed ratio, is the ratio of the angular velocity of the input gear to the angular velocity of the output gear. When using traditional (i.e., geared) transmissions, the gear ratio can be calculated directly from the numbers of teeth on the gears in the gear train. The torque ratio of the gear train, also known as its mechanical advantage, is determined by the gear ratio. The speed ratio and mechanical advantage are defined so they yield the same number in an ideal linkage. For a given transmission, the highest torque ratio or highest torque leverage occurs at the highest speed ratio.
Ball planet-planetary continuously variable transmissions (CVT) generally utilize a plurality of spherical rolling elements (also commonly referred to as ball planets, planets, or spheres) interposed between an input ring and an output ring, and in contact with a sun, to provide a continuous range of speed ratios. Ball planet-planetary CVTs may also be referred to as variators.
In some ball planet-planetary variators, the sun has two rolling elements, each in contact with the ball planets. It should be noted that contact between the two sun rolling elements and the ball planets may be direct contact, or there may be an intermediary fluid between the two sun rolling elements and corresponding points on the ball planets, which is commonly referred to as a traction fluid or friction fluid. Regardless of whether the contact is direct contact or there is an intermediary fluid, when a ball planet tilt angle is other than zero in a ball planetary variator and there are two sun rolling elements positioned with axial separation, the two sun rolling elements commonly rotate at different speeds. In other words, the surface speed of the ball planet at the contact position between the ball planet and the first sun rolling element is different than the surface speed of the ball planet at the contact position between the ball planet and the second sun rolling element. When both sun rolling elements are forced to rotate at the same speed, there exists sliding between the contacts. Sliding may limit the capacity for transferring power from the sun to the ball planets, affecting the power capacity or the efficiency of the variator. Sliding may also cause excessive noise, heat, vibration, abrasion or other negative effects, reducing the life of the variator.